It’s Complicated: Trees and Ecological Restoration

The best time to plant a tree was twenty years ago. The second best time is now.-Anonymous

Addendum: That is, unless the tree will grow just fine without your help or the tree doesn’t really belong there. In that case, the best time might be never.

Planting a tree is rejuvenating. It gets you outside, it’s good exercise, and it’s often good for the planet. Really, trees give us an awful lot and don’t ask for much in return. Among their many gifts are food, shade, animal habitat, building materials, erosion control, and fuel. Trees also filter our water and suck carbon out of the air. In cities, trees collect grit and grime that would otherwise coat our lungs.

But tree planting is not the same as restoration. Ecological restoration is the process of assisting the recovery of a damaged ecosystem. Trees are integral to many ecosystems, like forests, but to others they can be unwelcome. The relationship between trees and restoration is something like the relationship between fire and cooking. In the right context, the one can be very useful for the other. In the wrong context, it can make for a real disaster.

Pale purple coneflowers (Echinacea simulata) decorate a restored glade at Shaw Nature Reserve (Franklin County, Missouri). These plants prefer to grow in open sun or with just a few trees.

Here’s an example of trees getting in the way of restoration. In Missouri, glades are one of our characteristic ecosystems; they are rocky, open areas with sparse trees and diverse herbs like coneflowers, evening primrose, and various goldenrods. In some ways, they are like mini deserts, and many of them even have cacti and collared lizards. Glades are kept tree-less for a couple of reasons. First, the soil is too thin and dry to support many trees. Second, fires burn small trees and keep them from growing bigger.

Historically, many Missouri glades were maintained by fires started by indigenous people. Without fire, eastern red cedar trees come into glades, and as they grow their shade makes the habitat bad for light-loving herbs. Staff at Shaw Nature Reserve have made a big effort to restore glades by getting rid of red cedar trees. They can’t cut them down fast enough. The results are impressive. In some areas where eastern red cedar trees were removed, native plant diversity increased by 19% in two years.

Shaw Nature Reserve isn’t alone in battling trees to restore ecosystems. Throughout the world, many biodiverse grasslands, savannas, and woodlands are threatened by tree colonization, and the role of restoration is to remove them, or at least not plant more of them. Planting trees in such places does much more harm than good.

On the other hand, planting the right tree in the right place at the right time can be extremely useful. For example, in southern Costa Rica large areas of rain forest have been cut down and converted to cattle pastures. While cattle ranching provides income for many people, it also causes soil erosion and compaction. Sometimes cow pastures can grow back into rain forest on their own, and that is ideal. If rain forest grows back by itself it is less expensive and more natural.

Sometimes tropical rain forests recover by themselves, but when they don’t, tree planting can make a big difference. Left: A cow pasture in southern Costa Rica undergoing restoration. The hillside in the background has just been planted with trees. The area in the foreground was left to regrow naturally. Right: The same cow pasture, 9 years later. The planted trees have grown to about 40 feet tall. The area left to regrow naturally is still covered in pasture grasses. The yellow circles show a person, for scale. Images: Karen D. Holl.

In other cases, trees do not grow back quickly (or at all) and tree planting can help kick start forest recovery. Even small clusters of planted trees can attract native birds, increase seed dispersal, and create safe growing conditions for young trees. Planting trees can also create habitat for other species that depend on trees – such as orchids, bromeliads, and other vascular epiphytes. Tree planting is particularly vital for ecological restoration in harsh environments where trees don’t get many chances to grow up, such as in dry, coastal Peru or in highland Madagascar.

In ecological restoration, which tree species you plant also makes a big difference. In general, native trees provide a bigger ecological return on investment compared to non-native trees. One reason for this is that native trees support more insects, which are a food staple for our native birds. Some native trees (such as fig trees in Costa Rica) also make tasty fruits that could attract wildlife.

So should we plant trees to restore ecosystems?

Before we do, here are a few things we should consider:

(1) Do natural ecosystems in our area contain trees? If not, tree planting may do more harm than good.

(2) Are trees able to recover on their own without our help? If so, we may be able to save ourselves some effort by letting nature do the work.

(3) Which tree species will satisfy our goals? Picking the right tree for the right place will help us reach our goals more efficiently.